Re: FSGSBASE causing panic on 5.9-rc1

[Tom Lendacky]

On 8/20/20 10:10 AM, Sean Christopherson wrote:
> On Wed, Aug 19, 2020 at 05:21:33PM -0700, Andy Lutomirski wrote:
> > On Wed, Aug 19, 2020 at 2:25 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> > > On Wed, Aug 19, 2020 at 11:19 AM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:
> > > > On 8/19/20 1:07 PM, Tom Lendacky wrote:
> > > > > It looks like the FSGSBASE support is crashing my second generation EPYC
> > > > > system. I was able to bisect it to:
> > > > >
> > > > > b745cfba44c1 ("x86/cpu: Enable FSGSBASE on 64bit by default and add a chicken bit")
> > > > >
> > > > > The panic only happens when using KVM. Doing kernel builds or stress
> > > > > on bare-metal appears fine. But if I fire up, in this case, a 64-vCPU
> > > > > guest and do a kernel build within the guest, I get the following:
> > > >
> > > > I should clarify that this panic is on the bare-metal system, not in the
> > > > guest. And that specifying nofsgsbase on the bare-metal command line fixes
> > > > the issue.
> > >
> > > I certainly see some oddities:
> > >
> > > We have this code:
> > >
> > > static void svm_vcpu_put(struct kvm_vcpu *vcpu)
> > > {
> > >          struct vcpu_svm *svm = to_svm(vcpu);
> > >          int i;
> > >
> > >          avic_vcpu_put(vcpu);
> > >
> > >          ++vcpu->stat.host_state_reload;
> > >          kvm_load_ldt(svm->host.ldt);
> > > #ifdef CONFIG_X86_64
> > >          loadsegment(fs, svm->host.fs);
> > >          wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gsbase);
>
> Pretty sure current->thread.gsbase can be stale, i.e. this needs:
>
> 	current_save_fsgs();

I did try adding current_save_fsgs() in svm_vcpu_load(), saving the 
current->thread.gsbase value to a new variable in the svm struct. I then 
used that variable in the wrmsrl below, but it still crashed.

Thanks,
Tom

> 	wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gsbase);
>
> On a related topic, we really should consolidate the VMX and SVM code for
> these flows, they're both ugly.
>
> > >          load_gs_index(svm->host.gs);
> > >
> > > Surely that should do load_gs_index() *before* wrmsrl().  But that's
> > > not the problem at hand.
> > >
> > > There are also some open-coded rdmsr and wrmsrs of MSR_GS_BASE --
> > > surely these should be x86_gsbase_read_cpu() and
> > > x86_gsbase_write_cpu().  (Those functions don't actually exist, but
> > > the fsbase equivalents do, and we should add them.)  But that's also
> > > not the problem at hand.
> >
> > Make that cpu_kernelmode_gs_base(cpu).  Perf win on all CPUs.
> >
> > But I still don't see the bug.